Remove ubuntu 16 using gcc 7 as the environment only has gcc 9 (#219)

* Fix set ubuntu 16 gcc explicitly to gcc-9

See actions/runner-images#2950

.github/workflows/tec.yml

@@ -26,8 +26,6 @@ jobs:
           - os: ubuntu-16.04
             vcpkgCommitId: 'ec6fe06e8da05a8157dc8581fa96b36b571c1bd5'
             vcpkgTripletOs: linux
-            cc: gcc-7
-            cxx: g++-7
           - os: macOS-latest
             vcpkgCommitId: 'ec6fe06e8da05a8157dc8581fa96b36b571c1bd5'
             vcpkgTripletOs: osx
@@ -43,15 +41,15 @@ jobs:
       with:
         submodules: true
     - uses: lukka/get-cmake@latest
-    - name: Set compiler to gcc-7 on ubuntu-16.04
-      uses: lukka/set-shell-env@v1
-      if: contains( matrix.os, 'ubuntu-16.04')
-      with:
-        CXX: ${{ matrix.cxx }}
-        CC: ${{ matrix.cc }}
     - name: Install os deps (Linux)
       if: runner.os == 'Linux'
       run: sudo apt-get install libgl1-mesa-dev xorg-dev libglu1-mesa-dev libxinerama-dev libxcursor-dev p7zip-full -y
+    - name: Set gcc-9 on Ubuntu16.04
+      uses: lukka/set-shell-env@v1
+      if: contains( matrix.os, 'ubuntu-16.04')
+      with:
+        CC: gcc-9
+        CXX: g++-9
     - name: Set vcpkg's response file path used as part of cache's key.
       uses: lukka/set-shell-env@v1
       with: